hissing from behind hvac controls

my 97 just recently started hissing from behind the hvac control ****, the selecter that changes where the air comes out, it hisses until you get the engine under load then eventually goes away, i assume its a vacuum leak behind there, am i going to have to replace the whole control unit or could it be a vacuum line? also it still seems to work, just hisses, especially on the floor mode

The hissing will probably eventually get worse until you start to lose control of where the air comes out the vents. At minimum, pull off the control panel and see what you find. Unplug the connector where all the multi-colored vacuum lines plug into and see if there is any fluid or moisture in or around the holes. Those lines typically do not break down and leak, but if they get t-case fluid in them (read all about it here), they won't seal well at the connections, and you'll get some hissing.